The present invention relates to an ink-jet device capable of printing dots in tones and, more particularly, to a multi-drop driving ink-jet device in which the tone-level of each dot is determined by the amount of ink successively ejected as drops from a print head.
In a conventional ink-jet printer, a print head comprises a plurality of ink chambers separated from each other by partition walls formed of, e.g., electrostrictive members, and an orifice plate which covers the distal ends of the ink chambers and has a plurality of ink-jet nozzles. Ink in the ink chambers is ejected from the ink-jet nozzles by pressures applied upon deformation of the partition walls. Each partition wall deforms in accordance with the waveform of a drive pulse applied via an electrode. The waveform of the drive pulse is set to deform the partition wall in a predetermined sequence of causing the ink chamber to be temporarily expanded, then contracted, and returned to an original state.
In a multi-drop driving scheme, each ink-jet nozzle prints one dot by successively ejecting ink drops equal in number to the number of drive pulses supplied to the electrode. If there is no change in the waveform of the drive pulse, the tone-level of the dot increases almost in proportion to the number of ink drops.
When a dot pattern of a character or simple graphic is to be printed in monotone where no difference exists between the tone-levels of dots, a conventional multi-drop driving ink-jet printer generally prints all dots at the maximum tone-level in order to obtain a dot diameter which allows the dots to be adjacent to each other without any gap. In this case, drive pulses in the maximum number are successively supplied to an electrode, and ink drops in the maximum number are ejected from an ink-jet nozzle upon mechanical deformation of partition walls controlled by the drive pulse.
However, monotone printing is performed substantially in the same driving manner as for multi-tone printing, so the above ink-jet printer is inferior to an ink-jet printer dedicated for monotone printing in terms of the print speed. At the time of monotone printing, the number of ink ejecting motions may be reduced by, e.g., increasing the amplitude of the drive pulse to enlarge the ink drop. This requires a high drive ability of the drive pulse source.
It is an object of the present invention to provide an ink-jet device capable of performing monotone printing at a higher speed than for multi-tone printing without changing the drive ability of a drive pulse source in a multi-drop driving scheme.
According to the present invention, there is provided an ink-jet device which comprises a print head for ejecting ink as drops from an ink chamber to print a dot at a tone-level corresponding to an amount of ejected ink, and a drive unit for driving the print head by successively supplying a variable number of drive pulses each of which allows the ink chamber to generate pressure of ejecting a single drop of ink, wherein the drive unit includes a pulse generator for generating a fixed number of drive pulses and shortening the supply cycle of the drive pulses such that the pressure of the ink chamber is gradually increased to finally eject a drop of ink when the dot is printed in monotone.
With the ink-jet device, when a dot is printed in monotone, a single drop of ink is ejected from the ink chamber by shortening the supply cycle of the drive pulses. In this case, the dot can be printed in excellent contrast without requiring a longer period of time as compared with the case where a plurality of ink drops are ejected. In addition, such shortening of the supply cycle does not require an increase in the drive ability of the pulse generator.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.